I very much doubt Jeb Bush might run if his brother wins next month. That wouldn't just
look like a dynasty anymore.
If Kerry wins though, it's possible. Although 2012 is more credible than 2008. (Is there a term limit in Florida? Or could he keep on being Governor?)
If Kerry wins and the election is very close or decided irregularly again, perhaps GW would run again, especially if things don't go well for Kerry. GW could be the first person to both win without winning the popular vote, lose, and then win again.